A smoothing method for single crystal silicon carbide (SiC) especially with the combination of mechanical lapping and photocatalysis-assisted chemical mechanical polishing (PCMP) is proposed to solve the problems of traditional polishing methods, such as low efficiency, serious pollution, great damage, etc, by which the atomic smoothing of SiC wafer is removed by the powerful oxidability of UV photo-excited hydroxyl radical on nano TiO₂ particles. A orthogonal experiment (L₉(3³)) was first designed to develop efficient slurries for PCMP, and subsequently the optimized processing technology was obtained by method of detecting the surface roughness of silicon carbide wafer in the polishing process. The results show that the catalyst has greatest effect on the oxidability of slurry, and then is the electronic capture agent and the dispersing agent. The preferable slurry formula is TiO₂(0.5 g.L^-1), H₂O₂(1.5 mol.L^-1) and (Na₂PO₃)₆(0.1 g.L^-1). The proper polishing technique is the lapping SiC for 30 min with 5 μm and 2 μm diamond powders first, the material removal rate (R_MRR) is 8.72 μm/h and 4.56 μm/h, respectively. And then is the removing the damage caused by mechanical lapping with PCMP, 0.5 μm alumina powder was used for rough polishing for 50 min, and 0.05 μm alumina powder for fine polishing for 60 min in the polishing process, of which the R_MRR is 1.81 μm/h and 1.03 μm/h, respectively. The best surface roughness R_a with the proper progress can be reduced to about 0.47 nm, which may satisfy the requirements of high efficiency, ultra smooth and low damage for polishing single crystal silicon carbide.